The advent of electronic tip units allows the use of an electronic rating plug to set the circuit breaker ampere rating within a common-sized industrial-rated circuit breaker enclosure. Wherein earlier circuit breakers employing thermal-magnetic trip units were designed to meet the various circuit breaker ampere ratings by conforming the size of the circuit breaker components and the circuit breaker enclosure, a large variety of such circuit breakers were stored at the distribution site to meet the electrical distribution market requirements. U.S. Pat. No. 4,649,455 entitled "Rating Plug for Molded Case Circuit Breaker" describes one example of the use of an electronic rating plug to set the circuit breaker ampere rating.
U.S. Pat. No. 4,679,016 describes an industrial-rated circuit breaker design that is economically assembled in an automated process. The circuit breaker utilizes a thermal-magnetic trip unit to interrupt circuit current under overcurrent conditions within a protected circuit. To accommodate the various circuit breaker ampere rating requirements, the circuit breaker entails several sizes. It would be economically advantageous to use a common-sized enclosure with such a circuit breaker employing a thermal-magnetic trip unit in view of the lower cost with the thermal-magnetic trip unit per se.
The circuit breaker ampere rating is defined as the circuit current in amperes that the circuit breaker will continuously transfer without interrupting the circuit. The overcurrent interruption parameters are determined by the thermal-magnetic trip unit contained within the circuit breaker enclosure. Whereas the so-called "long time" and "short time" overcurrent circuit interruption is determined by the thermally-responsive element within the thermal-magnetic trip unit, the "instantaneous" circuit interruption is determined by the magnetic components therein. Earlier attempts to commercialize a thermal-magnetic circuit breaker having a mechanical rating plug have been hampered by the difficulties associated with maintaining the instantaneous circuit interruption response for the different ampere ratings. U.S. patent application Ser. No. 08-48,240 filed Nov. 8, 1993 entitled: "Mechanical Rating Plug for Thermal-magnetic Circuit Breakers" describes the operation of the thermally-responsive element within the circuit breaker to interrupt circuit current upon the occurrence of an overcurrent condition and the interaction between the mechanical rating plug and the thermally-responsive element to set the ampere rating.
Accordingly, one purpose of the instant invention is to provide a thermal-magnet trip unit for circuit breakers employing a mechanical rating plug within a common enclosure which duplicate the same thermal and magnetic response to overcurrent conditions as circuit breakers having enclosure that are sized to meet the different ampere ratings.